<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box{
            width: 100px;
            height: 100px;
            background: red;
            position: absolute;
        }
    </style>
</head>
<body>
    <div class="box"></div>
    <script>
        // 事件 ： 1.鼠标按下的事件 2.鼠标移动的事件 3.鼠标抬起的事件；
        var boxEle = document.querySelector(".box");
        boxEle.onmousedown = function(e){
            console.log("黑色的框");
            // boxEle.offsetLeft是不会改变的
            var x = e.clientX - boxEle.offsetLeft;
            var y = e.clientY - boxEle.offsetTop;
            boxEle.onmousemove = function(e){
                console.log("移动了");
                // 在移动的时候获取 offsetleft ，offset会不停的被设置
                var xx = e.clientX  ;
                var sx = xx - x;  // xx-(e.clientX-boxEle.offsetLeft)
                var yy = e.clientY;
                var sy = yy - y;
                // 设置left的值；
                boxEle.style.left = sx + "px";
                boxEle.style.top = sy + "px"
            }
        }
        boxEle.onmouseup = function(){
            this.onmousemove = "";
        }



    </script>
</body>
</html>